U.S. Pat. No. 6,731,018 discloses a device for capturing energy from tidal flows and river currents. This device includes a column fixedly mounted to the sea floor and a buoyant open bottomed tank slidably mounted on the column. Several hydrofoils are hingedly connected to the sides of the tank, each rotatable about a horizontal axis. The tank is driven in oscillatory vertical motion along the column by varying the angle of inclination of the hydrofoils with respect to the direction of water flow past the device. As the tank oscillates, the pressure of air within the tank varies. Ducts are provided in the tank and house high speed turbines for generating power in response to air flow generated by the changing pressure within the tank.
A disadvantage of the device disclosed in U.S. Pat. No. 6,731,018 is that it can not realign with changing directions of water flow. Another disadvantage of the device disclosed in U.S. Pat. No. 6,731,018 is that it inefficiently captures energy from the water flow, due to the required movement of a large tank against the resistance of the water. Another disadvantage of the device disclosed in U.S. Pat. No. 6,731,018 is that energy from the flow is transferred to the generators in an indirect manner, having to pass through multiple intermediate stages (water to hydrofoils, hydrofoils to tank motion, tank motion to air compression, air compression to flow across turbine), which together reduce the overall efficiency. Another disadvantage of the device disclosed in U.S. Pat. No. 6,731,018 is that the hydrofoils do not make optimal use of the fluid force imparted to them. A further disadvantage of the U.S. Pat. No. 6,731,018 device is that the hydrofoil shapes disclosed are not generally efficient in unsteady oscillating flows.
Further, the magnitude of tidal and marine flows vary depending on prevailing conditions and the stage in the tidal cycle. The flows can be very low in some situations and extremely high in others. A disadvantage of the device disclosed in U.S. Pat. No. 6,731,018 is that it inefficiently responds to changing magnitudes of water flow due to its inability to broadly vary its motions, as a result of its fixed alignment. The only adjustment that can be made to the U.S. Pat. No. 6,731,018 device is the hydrofoil angle. Accordingly, the range of conditions across which the U.S. Pat. No. 6,731,018 device may be operated effectively is quite restricted.